You gain Demonic Fury through damaging spells such as Corruption, Shadow Bolt etc. As your Demonic Fury increases, your damage increases. At any point you may activate Metamorphosis to greatly increase the damage increase from Demonic Fury (100% more benefit), but the duration of Metamorphosis only lasts as long as you have Demonic Fury - which in Meta form is used up by transformed versions of your abilities (ie: Shadow Bolt becomes Demonic Slash and Shadowflame becomes Cenarion Swarm).

This is just speculation however, we'll have to wait til the beta to know for sure :)